The present invention relates to a piezoelectric actuator which can be particularly Effused for actuating a fuel injector, as well as a method for manufacturing it.
Piezoelectric actuators, particularly for actuating fuel injectors, described in for example, German Patent Application No. 195 00 706 or German Patent No. 43 06 073. The piezoelectric actuators are composed of a plurality of piezoelectric layers stacked one over another which are each coated with an electrode on one surface. Usual piezoelectric actuators are composed of several hundred of piezoelectric layers stacked one over another in such a manner. In this manner, a relatively great length of actuating travel is achieved. As described in greater detail, for example, in German Patent 37 1 697, the electrodes of the individual piezoelectric layers must be alternately connected to a voltage source to generate an electrical field in individual layers which is oriented in the same direction. In the process, every other electrode is connected to a first pole of a voltage source, while the intermediate electrodes are connected to a second pole of a voltage source. Conventionally, as described in greater detail, for example, in British Patent No. 2 193 386, the electrodes, which extend up to the edge of the stacked piezoelectrical layers, are usually interconnected in the edge area on the outside. In a fully automated, large-scale manufacture, however, this contacting method requires considerable outlay, and is susceptible to faults, and, in addition, has the disadvantage that the electrodes, because they extend into the edge area, are not insulated against the surroundings so that the surface area of the piezoelectric actuators must be provided with an additional insulation.
The method according to the present invention has the advantage that it can be used in a fully automated manufacture, and results in very low manufacturing expenses. The electrodes of the actuator are not contacted on the outside, but on the inside, using an electrically conductive paste which is introduced into connecting openings. Therefore, the contacting according to the present invention is almost impervious to faults and insusceptible to external mechanical damage. The piezoelectrical actuators are manufactured in parallel with each other in a highly integrated manufacturing process. A block including a plurality of piezoelectric actuators is split up into the individual actuators only at the end of the manufacture. In this manner, the manufacturing rate can be considerably increased. The same advantages ensue also for the actuator according to the present invention.
Advantageous embodiments and improvements of the manufacturing process specified in claim 1 and the piezoelectric actuator specified in claim 10, respectively, are made possible by the measures characterized in the subclaims.
If an edge area of the actuators is left free of the electrodes, the advantage ensues that the electrodes are reliably insulated from the surroundings of the actuator. Therefore, no further measures for insulating the electrodes are required. Since, in addition, the electrodes are contacted via connecting openings inside the actuator, all live components are completely insulated toward the outside. The susceptibility to failure of the actuator is markedly reduced.
In the intermediate areas between the individual actuators, perforation holes can be formed in the foils which form the piezoelectric layers. On one hand, these perforation holes are used as degassing ducts during the subsequent firing of the stacked foils. On the other hand, the perforation holes make it easier to separate the stacked arrangement into the individual actuators. In this context, the separation can be carried out by applying an electrical field to the electrodes of adjacent actuators in an opposite direction to the polarity. While one adjacent actuator contracts, the other adjacent actuator expands. The resulting mechanical stress causes the actuators to break apart along the-separating line predefined by the perforation holes. However, the perforation holes make it also easier to separate the actuators by sawing along the perforation line predefined by the perforation holes. Another suited separating method is water-jet cutting.
The metallic electrodes can be advantageously applied using a screen-printing technique, vapor depositing, or sputtering, it being advantageous not to apply the electrodes all-over, but in a, for example, netlike pattern for reasons of material saving and better adhesion promotion to the ceramic layer lying above.